Conventional pressure controls for gaseous fuel systems, whether for vaporizing liquid LP or for dry gas at high pressures, typically use at least two pressure regulators in series. The upstream regulator, referred to as the primary, can be a pressure regulator that delivers fuel at pressures from a few inches of water to several pounds. The secondary regulator prepares the fuel to be received by the air. The secondary regulator can be a simple demand regulator that delivers fuel upon a vacuum signal through the fuel supply hose to a venturi device, or it may be built into the carburetor. The secondary regulator contained within a carburetor may be a simple demand regulator, or a charge forming device that reads the pressure drop across a venturi and provides a pressure feed to the main fuel orifice that is related to the venturi pressure drop, or an air/fuel proportioning device that opens both the air leg and the fuel leg upon the engine's demand, with the fuel leg completely closed at rest.
The predominant control configuration today combines the primary and secondary regulators within one housing, with or without vaporizer. The primary regulator uses a throttling valve controlled by a diaphragm to provide fuel at pressure to the secondary regulator of 2-8 PSIG and closes against the flow, so arranged that a pressure rise acting against the diaphragm will increase the closing force on the valve. The secondary regulator is a demand regulator whose valve closes against the flow with the diaphragm attached to the valve lever so that any delivery pressure rise greater than ambient will act to close the valve.
One system having a venturi carburetor uses a cantilever wire spring attached to the secondary valve lever that is adjusted with a screw to cause the secondary to pass gas for idle. A spring loaded vacuum responsive diaphragm is positioned below the secondary lever to overcome the cantilever spring closing the valve when the system is at rest.
The predominant control configuration is used to feed fuel to diaphragm controlled proportional mixers, proportional mixers that feed fuel below the throttle plate as well as all types of adapters and carburetors using venturis.
The vacuum at the outlet required to open the secondary regulator of 0.5-1.5 inches water is satisfactory with the diaphragm controlled mixers which require 6 inches water vacuum to open. The proportional mixers work well with this control, except with systems requiring full throttle starting, where even if the engine pulls 3 or 4 inches water vacuum at cranking the air/fuel mixture would be too lean to start.
The conventional control used with venturi systems must have an idle system with either an idle progression or an undersized venturi to eliminate a lean spot off idle and will never cold start without some type of assist.
The ideal control system would be one functional with all the systems in their simplest forms, and include the following features:
A. Complete control of fuel over the entire operating range. PA1 B. Starting ability at any throttle position when used with any type of gas carburetor or adaptor. PA1 C. The ability to trim the idle fuel with venturi systems. PA1 D. The ability to keep the fuel valve closed except upon engine demand.
The safety requirements for shut-down of a system require a fuel shut-off that must be opened by the engine's demand and must be arranged so that manual starting assist systems (hand and solenoid primers) cannot override this shut-down system. The accepted art is either a separate vacuum operated shut-off valve device which also houses the system's filter or a liquid solenoid valve that uses a vacuum switch or an oil pressure switch to prevent the flow of fuel with the ignition switch on when the engine is not running.
There have been attempts to incorporate a solenoid valve into the converter, but at light engine loads the vaporizing upstream of the solenoid frees the iron oxide power which accumulates in the magnetic field and restricts the valve opening.
Solenoid shutoffs are ideal for systems using two fuels (dual fuel) which require electronic fuel selection; the vacuum locks use a solenoid valve in the vacuum line for fuel selection when used on duel fuel systems.
The valve of a shutoff system should not be throttled against its seat to control the quantity of fuel required. Any contaminant unable to pass between the valve and its seat at shutdown is likely to become imbedded in the soft rubber valve when the valve receives the fuel closing load reducing the useful valve life.